Gynandromorphs are mutations of sexually dimorphic organisms that simultaneously exhibit male and female characteristics. They are also sometimes known as half-siders because this mutation typically presents with half the body as female and the other half as male, split ventrally down the middle. It occurs as a result of errors during embryonic development and is extremely rare. Such organisms are a topic of much interest for researchers.

Insects are most likely to exhibit this unusual trait because of the way they develop. In butterflies, gynandromorphs can be particularly striking, as male and females of the same species often look radically different. The mutation arises as an error in the first cell division. Instead of passing on a complete set of sex chromosomes, the divided cell passes on only a partial set to one half, causing it to develop different physical characteristics. Non-disjunction, as the failure of a chromosome to separate fully during cell division is known, is usually fatal for the cell because it needs that genetic material, but every now and then the result is a gynandromorph.

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This mutation has also been observed in crabs and lobsters. As early as the 1700s, scientists remarked on unusual specimens that appeared to be half female and half male, and speculated on the causes. It has also been documented in chickens, where it appears to be the result of double fertilization. Researchers on the fruit fly, a favorite subject of geneticists, have used a number of experimental conditions to learn more about how and why gynandromorphy develops in some organisms.

An examination of genetic material from gynandromorphs shows that the male side and female side are identical, except for the difference in the sex chromosomes. These organisms may have dramatic variations in their coloring and patterns and can also develop external structures like combs in chickens. Samples of butterflies with this trait are in storage facilities at some museums and collections as a topic of general scientific interest.

Gynandromorphy has not been observed in mammals. This trait develops only in very controlled conditions, and the development of gynandromorphs in nature is extremely rare. Some biology texts provide images of particularly interesting examples, and it is also possible to find pictures in scientific papers. One consequence of research in the lab on gynandromorphs has been a deeper understanding of the function of sex chromosomes in the development of the brains and reproductive tracts in some organisms.

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